PROFESSOR: Hi everyone.
Welcome to the biochemistry help sessions.
This is the covalent bonds, chirality, electronegativity,
and hydrogen bonds module.
In this help session, I'll be going over the first practice
problem in this module.
If you have not had a chance to look over this problem
yourself, please go back, look over the problem, try it, and
then come back and we'll go through it together.
Now that you've had a chance to look at the problem, let's
get started.
First of all, what is this molecule?
To me, this molecule looks like it's a sugar.
It's a polysaccharide, to be exact, and it's actually a
chain of glucose molecules.
So in this problem, the first part of the problem asks us to
name each circled bond in this molecule.
So the first circled bond is right here, and this is a
carbon to carbon bond.
Now, the first thing that we can notice about this bond is
that it's a carbon to carbon bond, and we know that carbon
only forms covalent bonds in biological systems.
So the first thing we can say is that this bond is a
covalent bond.
Second, we need to decide if this bond is
polar or non polar.
And the way we decide that is we look at the
electronegativity of the atoms involved.
In this bond we have two carbons.
And so since they're the same atoms, we know that they have
the same electronegativity.
And bonds between two atoms of the same electronegativity are
non polar bonds.
So we know that this carbon to carbon bond is
covalent and non polar.
Now we'll look at the second bond circled.
This bond is a carbon to oxygen bond.
It is similar to the first bond in that it involves
carbon, so we can say that it is a covalent bond.
What's different about this bond, however, is that it is a
carbon bound to an oxygen.
And when you look at the electronegativities of carbon
and oxygen, you find that oxygen is much more
electronegative than carbon.
So in a carbon to oxygen bond, you actually have oxygen is
greedy, and it's going to take the electrons closer to itself
and not share them equally with the carbon.
So in this kind of bond, oxygen will be slightly
negative and carbon will be slightly positive, because the
electrons are attracted closer to the oxygen.
In a bond where you have a slightly negative side and a
slightly positive side, we call this a polar bond.
So we can say that a carbon to oxygen bond is a polar bond.
So in this first part, we learned that the difference
between a covalent and a non polar and a
covalent polar bond.
The second part of the problem, part B, asks if this
sugar molecule is soluble in water.
So first you need to know whether this molecule is polar
or non polar.
And since we know that it has carbon to oxygen bonds, we
know that it has polar bonds, so it is a polar molecule.
And what do we know about water?
We know water is an oxygen bound to two hydrogens.
And since it has this very electronegative oxygen, we
know that it is also a polar molecule.
And an easy rule of thumb is that like dissolves like.
So since these are both polar molecules, sugar dissolves
very easily in water.
If you had a fat, on the other hand, which is a non polar
molecule, it would not dissolve in water.
That's why when you put oil and water, you
don't see it dissolve.
So now that we understand why sugar dissolves very easily in
water, we'll move into the third part of the problem,
where they ask what types of bonds can form between these
long polymers.
So looking at these polymers, the first thing that we can
notice is that there are these oxygens and hydrogens on each
of the polymers.
And looking at these, we can see how you can have hydrogen
bonding, which we learned about in class, between the
oxygen of one sugar molecule to the hydrogen of another
sugar molecule, thus forming hydrogen bonding networks
between the sugar molecules.
And in this way, they can interact with each other.
So in this help session we have covered the difference
between a covalent non polar bond and a
covalent polar bond.
We have covered why water can dissolve sugar very easily,
and what kind of bonds can form between
these two sugar molecules.
Thank you.